844 



KESPIKATION IN LOWER ANIMALS 



of soft skin (in the clam two on each side of the body) 

 called gills. These gills are thin-walled membranes or 



extensions of the skin which 

 cover the body of the clam, 

 and thus the water in which 

 they lie and which passes over 

 them is separated from the 

 blood by only this thin skin 

 and the thin walls of the ves- 

 sels. Oxygen is able to pass 

 easily through these mem- 

 branes and enter the blood 

 just as it does through the air- 

 sac linings and capillary walls 

 of our lungs. The blood of the 

 clam contains no corpuscles 

 and no haemoglobin and is 

 colorless. It does, however, 

 contain a substance similar 

 to haemoglobin called haemo- 

 cyanin, which takes up the oxygen and distributes it. At 

 the same time the blood passes its carbon dioxide out to 

 the water and is thus aerated. This aerated blood passes 

 from the gills back to the heart, and is then pumped over 

 the body to the tissues, finally returning to the gills, and 

 so on indefinitely. 



Here in the clam, then, we have a definite structure 

 developed solely for respiratory purposes, and showing 

 practically the same relation between blood and the oxygen- 

 supplying medium as that seen in our air sacs. The clam, 

 also, has a pair of tubes which cause a constant stream of 

 water to pass into and out of the shell and over the gills, 



Fig. 156 — Diagrammatic section of 

 a clam (Anodonta); a, lobes 

 of mantle ; b, gills, showing 

 transverse partitions ; c, ventricle 

 of heart ; d, auricles ; e, pericar- 

 dium ; /, g, kidneys ; h, venous 

 sinus ; k, foot ; A, branchial, or 

 pallial, chamber ; B, epibranchial 

 chamber. 



